3.6 Conclusions

Of the ocean–atmosphere–land three-component system, ocean contains
by far the most natural carbon. There is no realizable physical limit to the
uptake capacity of the ocean and it is estimated that on millennial timescales
the ocean will ultimately store up to 90% of the CO2 released by human activity.
However, on timescales more relevant to human society, the uptake rate of CO2
is controlled by a complicated matrix of physical, chemical and biological
processes. Studies suggest that the ocean has been the primary sink for excess
CO2 released to the atmosphere over the last 200 years, but the ocean’s
role may be changing over the next few decades to centuries.

Because the anthropogenic signal in the ocean is relatively small compared
to the natural background concentrations and relative to the observed seasonal
to interannual variations, it has been difficult to directly quantify the
uptake and storage of anthropogenic CO2 in the ocean. This has been further
hampered by a paucity of data. The current estimates have been based primarily
on indirect approaches or on a number of simplified assumptions, ignoring
a number of potential carbon cycle and carbon–climate feedbacks. The potential role of these
feedback processes in the ocean carbon cycle is just beginning to be understood
and fully appreciated. As we obtain more data on processes and improve their
representation in models, we will be better equipped to estimate the long-term
role of the ocean in the global carbon cycle and its impact on future climate
change.